The present invention relates generally to a portable radio terminal. More specifically, the present invention relates to a method for constructing a linear antenna provided at a position which is offset from the longitudinal center line of a housing.
In general, portable radio terminals represented by PDCs (Personal Digital Cellular systems) are used for carrying out voice conversations, and are often held in any hand of a talking person near the external ear capsule of the head of the human body to be used. There are some cases where the transmitting/receiving characteristics of radio waves on the antenna vary in accordance with which hand of the talking person holds a radio terminal. First, this phenomenon will be described below.
FIGS. 1A and 1B are illustrations showing the difference in position of an antenna 3 with respect to a housing 2 in accordance with which hand of a user holds a conventional portable radio terminal 1. FIG. 1A shows a case where the portable radio terminal 1 is held in the user's left hand, and FIG. 1B shows a case where the portable radio terminal 1 is held in the user's right hand. As shown in FIGS. 2A and 2B, the portable radio terminal 1 is generally inclined at, e.g., about 60°, with respect to the vertical direction to be used.
As can be clearly seen from FIGS. 1A through 2B, the relative position of the antenna 3 with respect to the housing 2 is different between the case where the portable radio terminal 1 is held in the left hand as shown in FIGS. 1A and 2A and the case where it is held in the right hand as shown in FIGS. 1B and 2B. When the portable radio terminal is held in the left hand as shown in FIGS. 1A and 2A, the antenna 3 is arranged on the lower side of the housing 2 than that when the portable radio terminal is held in the right hand as shown in FIGS. 1B and 2B. Assuming that the state shown in FIGS. 1A and 2A is a state that the antenna is arranged on the lower side and that the state shown in FIGS. 1B and 2B is a state that the antenna is arranged on the upper side, these states will be described below. That is, the portable radio terminal 1 is used in a state that the position of the antenna varies in accordance with which hand of the user holds the portable radio terminal 1.
In portable radio terminals, it is known that the radiation of radio waves from the housing 2 itself can not ignored. In particular, a high-frequency current is greatly distributed in the upper portion of the housing 2 near the feeding point for the antenna 3, and there are also some cases where the radiation of radio waves from the high-frequency current serving as a wave source is equal to the radiation from the antenna 3. That is, as shown in FIGS. 3A and 3B, the radiation of radio waves from the portable radio terminal 1 is carried out by the vector sum of the high-frequency currents on the antenna 3 and the upper portion of the housing 2. Since the relationship between the high-frequency currents and the radiation field is a proportional relationship, the high-frequency current will be discussed below.
FIGS. 3A and 3B show a vector sum when the current on the upper portion of the housing 2 is substantially equal to the current on the antenna 3. As can be clearly seen from FIGS. 3A and 3B, in the case of the holding in the left hand shown in FIG. 1A, the vector sum extends substantially in a horizontal direction as shown in FIG. 3A, and in the case of the holding in the right hand shown in FIG. 1B, the vector sum extends substantially in a vertical direction as shown in FIG. 3B. That is, the polarization characteristic, which is an important point of the characteristics of the antenna, is greatly different between the holding in the right hand and the holding in the left hand.
In portable radio terminals, the antenna of the base station often has the vertical polarization. Therefore, in order to enhance the polarization efficiency between the antenna of the terminal and the antenna of the base station, it is desired that the antenna of the terminal also has the vertical polarization. In the cases shown in FIGS. 3A and 3B, the polarization in the case of the holding in the right hand shown in FIG. 1B is closer to the vertical polarization than that in the case of the holding in the left hand shown in FIG. 1A, so that the holding in the right hand is desired as the characteristics of the antenna. Therefore, even if the antenna is arranged so as to be offset in the hope that the terminal is held in the right hand as shown in FIG. 1B, if the hand holding the terminal is changed, the polarization of the antenna 3 is close to the horizontal polarization shown in FIG. 1A, so that it is clear that the polarization efficiency remarkably deteriorates.
In the above description, the antenna 3 is mounted on the housing 2 so as to be offset downwards when the terminal is held in the left hand. However, this is the same if the antenna 3 is mounted on the housing 2 so as to be offset upwards when the terminal is held in the left hand. In this case, the characteristics of the antenna that the polarization efficiency in the case of the holding in the left hand is higher than that in the case of the holding in the right hand are enhanced. However, also in this case, if the portable radio terminal is held in the right hand, the polarization of the antenna is substantially the horizontal polarization that the antenna is arranged on the lower side, so that the polarization efficiency deteriorates.
The reason why the characteristics in the case of the holding in the left hand are different from those in the case of the holding in the right hand is that the portable radio terminal 1 is asymmetric. Therefore, by arranging the antenna 3 at the center of the portable radio terminal so that the whole portable radio terminal 1 including the antenna 3 is symmetric, the difference in characteristics due the holding hand can be removed. However, if the antenna 3 is provided at the center, there is the following problem.
In general, the antenna 3 of the portable radio terminal 1 is extensible in the longitudinal direction of the housing 2, so that the antenna 3 can be housed in the housing 2. When the antenna 3 is housed in the housing 2, if the antenna 3 is arranged at the center, a space for housing the antenna 3 must be ensured in the central portion of the terminal 1. However, a radio circuit and battery for a radio transmitter (not shown) are arranged in this portion, in which it is difficult to simply ensure the space for housing the antenna. Therefore, it is not realistic to provide the antenna housing space at the center of the housing 2.
Thus, in conventional portable radio terminals, there is a problem in that the polarization characteristics vary in accordance with which hand holds the terminal 1, so that the polarization efficiencies of radio waves from the base station having the vertical polarization characteristics and the antenna remarkably deteriorate to make communication impossible. Furthermore, the antenna of the base station must emit uniformly over 360 degrees on the horizontal plane, so that the use of an antenna having the vertical polarization is by far effective in view of the decrease of the costs for the antenna.
As described above, in the antenna for use in the conventional portable radio terminal, the difference in quality of the performance of the antenna is caused by the difference in polarization characteristics of transmitted/received radio waves between when the terminal is held in the user's left hand and when the terminal is held in the user's right hand, so that there is a problem in that the portable radio terminal can not optimally be operated according to the state that the terminal is held.